US5170621A - Flame failure detection - Google Patents

Abstract

A dual rotor gas generator of a gas turbine generator is fired with fuel delivery based on a fuel demand signal. Actual air flow is determined by sensing the low pressure rotor speed. A ratio of the fuel demand and actual air flow is calculated. Actual fuel flow is not used. Loss of flame is detected based on a fuel/air ratio as compared to a preselected value.

Description

TECHNICAL FIELD

The invention relates to gas turbine power generators and in particular to land based turbines.

BACKGROUND OF THE INVENTION

In gas turbine engines there is no independent supply or control of airflow to the combustor. Firing the combustor drives the turbine which is direct connected to the compressor, thereby supplying the airflow.

During load operation fuel is input to maintain a controlled power output, speed and/or temperature The fuel air ratio is dictated by the gas turbine design, varying somewhat through the load range and during transients. An increase in fuel flow increases gas temperature, initiating a turbine speed increase. This increases the compressor speed providing an increased airflow. The rate of change of fuel flow is limited to, among other things, avoid excessive swings in fuel air ratio.

In the event of a flameout in a combustor, fuel continues to be introduced by the control system in an attempt to meet demand. Airflow continues as the unit coasts down. This introduction of unburnt fuel into the air produces an explosive mixture which could be set off by any hot spot or spark. The potential magnitude of any explosion is a function of the amount of fuel present and the stoichiometry of the mixture.

Flame scanners in gas turbine engine combustors are not desirable because of sooting problems and background incandescence of materials inside the combustor and turbine area. A simple reliable means for detecting a flameout and effecting a timely trip of the fuel is desired.

SUMMARY OF THE INVENTION

The conventional land based gas generator has a high and low compressor connected to a high and low turbine, respectively. Fuel is fired into the combustor between the compressors and turbines and a free power turbine is located downstream of the low pressure turbine.

Airflow sensing means sense a measure of the actual airflow, this airflow sensing means preferably being a sensing of the speed of the low pressure compressor. Conventional control means exist for establishing a fuel demand signal in response to the various factors to be controlled. A valve responds to this fuel demand signal to supply fuel to the combustor.

A calculating means calculates the ratio of fuel demand to actual airflow. In response to this ratio reaching a preselected value, it is assumed that a flame failure has occurred and accordingly the fuel shut off valve is tripped. This trigger ratio may vary in the load range and in particular may be different between below idle and above idle speeds.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic of a gas turbine engine system;

FIG. 2 is a control schematic of the fuel control; and

FIG. 3 is a schematic of the flameout detection logic.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring to FIG. 1, thegas generator 10 includes a low pressure rotor comprised of alow pressure compressor 12 direct connected tolow pressure turbine 14 Also, a high rotor with ahigh pressure compressor 16 is directly connected tohigh pressure turbine 18. A plurality ofcombustors 20 receive fuel fromfuel supply line 22 with the flow being modulated bycontrol valve 24 driven byactuator 26.Fast operating valves 28 and 30 may be closed for the purpose of tripping fuel byactuators 32 and 34. Apower turbine 36 is located downstream of thelow pressure turbine 14 and is direct connected to apower generator 38. Waste heat steam generator 40 receives the exit gas flow from the gas turbine for the generation of additional power.

Referring to FIG. 2, high pressurerotor speed control 42 sends a signal throughline 44 indicative of fuel demand.Power turbine control 46 sends a signal also indicative of fuel demand throughcontrol line 48. Exitgas temperature control 50 sends a similar signal throughline 52. Theacceleration limit control 54 sends a fuel demand signal throughline 56 while low pressurerotor speed control 58 sends such a signal throughcontrol line 60.

These signals are all sent to low signal select 62 where the low signal is selected and sent throughcontrol line 64 to high signal select 66.

Also entering high signal select 66 is a signal fromminimum flow schedule 68 throughline 70 and a signal fromdeceleration control 72 throughline 74. The selected high signal indicative of fuel demand is sent throughcontrol line 76. Emergency shutdown indicated bylogic box 78 may be initiated bymanual trip line 80 orcontrol trip line 82 which receives signals from the various control loops indicated to an extreme condition dictating a trip.

Since the gas turbine is more responsive to fuel increases at high loads than at low loads, avariable gain 84 is applied to the fuel demand signal throughline 76 producing a modified fuel demand signal inline 86. This signal is substantially the square of the unmodified signal inline 76.

This gas turbine is arranged to burn either liquid fuel, gaseous fuel, or a combination of the two. It has been found effective to set the fuel demand signal inlines 76 and 86 as based on a weight flow of liquid fuel. Appropriate modifications of this signal are made insimultaneous burn logic 88 which send appropriate signals to gas fuelfinal driver 90 and liquid fuelfinal driver 92. Watersteam injection logic 94 introduces water or steam along with the fuel in response to the exit gas temperature to achieve increased power in accordance with well known gas turbine principles. While this increases the power output it does also increase the probability of flameout. Flameouts, of course, may also occur from other triggering situations.

In the event of a flameout, regardless of which of the control loops is dictating operation at the time, there will be an instantaneous demand for an increased fuel flow. It is this demand signal rather than any actual fuel flow, or response of a fuel flow valve, which is used as a basis for triggering the flame failure detection. The signal obviously leads any response to the signal by the fuel valves, and functions regardless of any possible jamming of the valve or blockage of the fuel flow path. For the purpose of the logic offlameout detector 96, a signal indicative of the fuel demand is sent throughcontrol line 98.

As shown in FIG. 3, this fuel demand signal passes todivision function 100. A measure of airflow also passes to this division signal. Preferably asignal 102 indicative of the low pressure speed is used which in accordance withlogic box 104 establishes an actual airflow as a function of the rotor speed. This actual airflow signal passes throughcontrol line 106 where the fuel/air ratio is calculated inbox 100 resulting in a fuel/air ratio signal inline 108.

Using a heating value of the fuel of 18,500 BTU's per pound and a required air for combustion of 755 pounds per million BTU, the stoichiometric fuel/air ratio in weight units is 0.072. During start the gas turbine overall fuel/air ratio is set to operate at 0.028 fuel/air ratio while at idle this ratio decreases to 0.008. At full power the unit operates at a fuel air ratio of 0.024. Considering ordinary deviations in the operation and transient conditions, the fuel/air ratio should never exceed 0.034 in normal operation nor 0.070 below idle.

Therefor, while monitoring the fuel demand signal and a measure of the actual airflow, the ratio is continuously watched. Below idle, if this ratio exceeds 0.070, or if above idle the ratio exceeds 0.034, a flame failure exists and the quick closing fuel valves are tripped.

Response time for such operation has found to be on the order of 0.6 seconds. The use of the demand signal leads the actual fuel flow, and the amount of fuel injected into the injector during a short period will not lead to a significant explosive mixture.

Referring again to FIG. 3, thelimit selector 110 receives a signal through line 112 indicating whether operation is above or below idle. The appropriate limit, either 0.034 or 0.070 is sent throughline 114 tocomparison logic 116. Should the fuel/air ratio signal inline 108 exceed the limit set inline 114, a trip signal 118 is sent to trip the fast operating trip fuel valves.

Claims (4)

We claim:

1. A land based gas turbine engine comprising:

a compressor;

a combustor connected to receive air from said compressor;

a gas turbine connected to receive gas from said combustor and being shaft connected to said compressor;

a free power turbine receiving gas from said gas turbine;

a fuel trip valve;

airflow sensing means for sensing a measure of actual airflow;

control means for establishing a fuel demand signal;

modulation means for supplying fuel to said combustor in response to said fuel demand signal;

calculating means for calculating a ratio of the fuel demand and the actual airflow; and

actuator means for closing said fuel trip valve in response to a preselected value of said ratio of said fuel demand and said actual airflow.

2. A gas turbine engine as in claim 1:

said compressor comprised of a low pressure compressor and a high pressure compressor;

said gas turbine comprised of a low pressure turbine connected to said low pressure compressor and a high pressure turbine connected to said high pressure compressor; and

said airflow sensing means comprising means for sensing the speed of said low pressure compressor.

3. A gas turbine engine as in claim 1: said control means including:

a variable gain signal modifier producing a modified demand signal; and

means for directing said modified signal to said calculating means.

4. A gas turbine engine as in claim 2: said control means including:

a variable gain signal modifier producing a modified demand signal; and

means for directing said modified signal to said calculating means.

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